If your game runs slow, get out a profiler to see what actually takes the time and then optimize these hotspots. Everything else is just wasted time.
And most of the time that "hotspot" will be rendering.
if your game is a relatively small project and unless you do something immensely stupid you will probably hit gpu bottlenecks long before cpu bottlenecks. And that will also probably be something stupid or unecessary(too much polygons or complex effects for scenery fillers/small props, missing lods etc. or I/O can cause hiccups when loading large shit on runtime)
